I would ring and ask. I’m sure they would let him in, my partner has came in the changing rooms before to help me with baby and then sat on the side by the pool, or watched through the window in the cafe if in the Lagoon pool xx
Isleworth leisure? We go there all the time and it's mixed changing. Normally me and my partner take our little one and we all go in together to change x
Not been for a couple of years but they used to have mixed pool changing with family cubicles. Can’t remember on the spectator seating though, sorry!